Strange behavior with frozen column

I use the following code snippets below but can't duplicate this issue. Are you using a customized header?

var dsOption= {

fields :[
		{name : 'no'  },
		{name : 'country'  },
		{name : 'administrator'  },
		{name : 'employee'  },
		{name : 'bill2005' ,type: 'float' },
		{name : 'bill2006' ,type: 'float' },
		{name : 'bill2007' ,type: 'float' },
		{name : 'bill2008' ,type: 'float' },
		{name : 'orderDate' ,type:'date'  }
		
	],

recordType : 'json',
	data : __TEST_DATA__
}

var colsOption = [
     {id: 'no' , header: "Order No" , width :60  },
     {id: 'employee' , header: "Employee" , width :80  },
	   {id: 'country' , header: "Country" , width :70  },
	   {id: 'administrator' , header: "Administrator" , width :80, frozen:true  },
	   {id: 'bill2005' , header: "2005" , width :60},
	   {id: 'bill2006' , header: "2006" , width :60},
	   {id: 'bill2007' , header: "2007" , width :60},
	   {id: 'bill2008' , header: "2008" , width :60},
	   {id: 'orderDate' , header: "Delivery Date" , width :100}
       
];

var gridOption={
	id : grid_demo_id,
	width: "700",  //"100%", // 700,
	height: "350",  //"100%", // 330,
	container : 'gridbox', 
	replaceContainer : true, 
	
	dataset : dsOption ,
	columns : colsOption,
	toolbarPosition : null
};

var mygrid=new Sigma.Grid( gridOption );
Sigma.Util.onLoad( Sigma.Grid.render(mygrid) );

and data sample is

var __TEST_DATA__=
[

{no: "010-0",sequence:1,country:"MA",administrator:"Jerry",employee:"Keith",bill2005:50,bill2006:57,bill2007:80,bill2008:46,orderDate:"10/02/2008"},
{no: "010-1",sequence:1,country:"SP",administrator:"Charles",employee:"Marks",bill2005:79,bill2006:37,bill2007:40,bill2008:90,orderDate:"10/28/2008"},
{no: "010-2",sequence:1,country:"SP",administrator:"Vincent",employee:"Harrison",bill2005:91,bill2006:75,bill2007:31,bill2008:40,orderDate:"10/02/2009"},
{no: "020-3",sequence:1,country:"RA",administrator:"Edward",employee:"Sidney",bill2005:61,bill2006:31,bill2007:80,bill2008:47,orderDate:"10/02/2012"},
{no: "020-4",sequence:1,country:"CA",administrator:"Patrick",employee:"Solomon",bill2005:82,bill2006:70,bill2007:33,bill2008:38,orderDate:"10/02/2123"},

{no: "010-0",sequence:2,country:"MA",administrator:"Jerry",employee:"Keith",bill2005:50,bill2006:57,bill2007:80,bill2008:46,orderDate:"10/02/2008"},
{no: "010-1",sequence:2,country:"SP",administrator:"Charles",employee:"Marks",bill2005:79,bill2006:37,bill2007:40,bill2008:90,orderDate:"10/28/2008"},
{no: "010-2",sequence:2,country:"SP",administrator:"Vincent",employee:"Harrison",bill2005:91,bill2006:75,bill2007:31,bill2008:40,orderDate:"10/02/2009"},
{no: "020-3",sequence:2,country:"RA",administrator:"Edward",employee:"Sidney",bill2005:61,bill2006:31,bill2007:80,bill2008:47,orderDate:"10/02/2012"},
{no: "020-4",sequence:2,country:"CA",administrator:"Patrick",employee:"Solomon",bill2005:82,bill2006:70,bill2007:33,bill2008:38,orderDate:"10/02/2123"}

];